In many processes, two or more liquids come into contact with one another. Azeotropes of these liquids may form and simple separation methods are often inadequate to completely separate the liquids. This can cause significant problems in a variety of chemical or measurement processes. For example, the presence of water can cause: interferences associated with measurements using infrared (IR) or Nuclear Magnetic Resonance (NMR) spectroscopy; increases in corrosion rates in metalst or deactivation of catalysts in manufacturing processes. Certain portions of conventional processes, such as product recovery and reactant or solvent recycling, require separation of the liquids. Some separation methods commonly used are distillation, the use of dessicants, and the use of molecular sieves. However, these and other separation methods often have limited efficiency, require complex and costly equipment, or involve several processing steps.
Pervaporation, a process of selective permeation through a membrane and subsequent evaporation, is quite popular for use in separating and purifying liquids and gases. The recent surge in membrane technology has resulted in the availability of numerous polymer materials with an assortment of transport properties. Some of the more frequently used materials are sulfonated halopolymers, or metal salts thereof. A common sulfonated halopolymer is a polymer of perfluorosulfonic acid (PFSA). Examples of such polymers, and methods of preparing such polymers, can be found in the following U.S. Pat. Nos. that are incorporated herein by reference: U.S. Pat. Nos. 3,282,875; 3,560,568; 4,025,405; 4,116,888; 4,123,336; 4,126,588; 4,178,218; 4,209,635; 4,270,996; 4,329,435; 4,330,654; 4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412; 4,358,545; 4,417,969; 4,462,877; 4,470,889; and 4,478,695. See also T. D. Sierke, "Perfluorinated ionomer Membranes", ACS Symposium Series No. 180, pp. 386-88 (1982). For a discussion of the most commonly preferred embodiments of PFSA polymers, See De Veilis et al., U.S. Pat. No. 4,846,977, col. 5, lines 1-36 (incorporated herein by reference).
Pervaporation devices, which are based on the selective permeation of mixture components through permselective membranes, have been developed and modified for use in many different applications. For purposes of this invention, "pervaporation device" and "permeation device" are used interchangeably to mean the same thing. Skarstrom et al. (U.S. Pat. No. 3,735,558), for example, disclose a process and apparatus for separating selected or key components from mixed fluid feeds using a PFSA membrane. They employ a combination of pressure and concentration gradients in a continuous process to achieve this purpose. More specifically, they create both a pressure difference and a composition difference between two counter-currently flowing fluid streams separated by the walls of a hollow PFSA membrane. These pressure and composition differences are also referred to as pressure gradients and concentration gradients. They act as driving forces to induce select or permeable fluid components of the feed to flow from a high pressure region that contains feed, through semi-permeable walls of the PFSA membrane to a low pressure region that contains either reflux, purge fluid, or both, thereby substantially removing selected permeable fluid components from the feed.
Another pervaporation method and device are disclosed by De Vellis et al. in U.S. Pat. No. 4,846,977, the teachings of which are incorporated herein by reference. A mixture comprising one or more polar liquids and one or more non-polar liquids is placed in contact with one side of a membrane comprising a polymer of a PFSA, or metal salt thereof. On the other side of the membrane, conditions are provided such that polar liquids which have permeated through the membrane are carried away from the membrane. This is done by providing either a desiccant, a vacuum and purge gas, or both. Both the non-polar liquids and the fluid desiccant containing the permeated polar liquids are then separately removed from the vicinity of the membrane. Although this method and device function well, it would still be desirable to increase their efficiency.
In relation to a liquid chromatography method and apparatus, Stevens et al. (U.S. Pat. No. 4,751,004) disclose an improved flow-through reactor in which the improvement comprises a combination of a packing means and a membrane. They teach that packing an ion exchange membrane or membrane channel, used for ion chromatography, with inert or charged ion exchange beads increases overall suppressor efficiency. They use "suppressor efficiency" to refer specifically to the efficiency of reactions that are distinctive to ion chromatography. In these reactionst an electrolyte eluent is converted to a weakly ionized form in order to sensitively detect sample ions in a low conductive background. They attribute the increase in efficiency to the mixing action of the packing on ionic solute as it flows through the membrane channels, resulting in a reduction in laminar flow of the solute. See col. 7, lines 17-34 (incorporated herein by reference).
In order for an apparatus to properly function as a pervaporation device, though, the membrane must be effectively sealed to the device. Several means of sealing the membranes to pervaporation devices are known. For example, Skarstrom et al., col. 13, lines 20-44, use headers, end seals, and clamps. These sealing components are typically constructed of non-membrane materials such as epoxy resinst polyamide, silicone (siloxane polymers), polyethylene, polypropylene, butyl rubbery neoprene, polyester, or fluorocarbon polymers. De Veilis et al. (U.S. Pat. No. 4,846,977), col. 12, lines 45-50, adhesively pot a PFSA membrane into two lipped epoxy tubesheets. The lip on each one of the potted epoxy tubesheets allows the formation of a seal with a casing face plate. For examples of potting materials, see De Veilis et al., col. 14, lines 44-51, and James C. Davis and Dennis P. Petersony "Hollow Fiber Postcolumn Reactor for Liquid Chromatography," 57 Anal. Chem. 770 (1985). Other means of sealing include friction fit slip seals and swell seals wherein, typically, an end of the membrane is forced or swelled over a piece of the device.
There are, however, many problems associated with the above described sealing means. For example, many pervaporation device seals cannot withstand pressures suitable for effective permeation process conditions. The seals can also restrict effective flow of the fluid through the membrane. This is especially apparent when a sealing means component that is composed of a material different from the membrane (hereinafter referred to as a non-membrane component) interacts chemically with a solvent. Upon chemical interaction of a solvent and a non-membrane component, the component may swell and eventually rupture the membrane. Sealing means of this type are prone to leaking and typically last only a short time. In addition, many pervaporation device seals are incapable of being changed or modified quickly. It would be desirable to have a sealing means that substantially reduces these above-described problems.
A possible method of sealing a membrane to a pervaporation device would be to permanently and functionally deform the membrane into its desired sealing position with the device. An example of this would be a membrane that is permanently flanged and then functionally fitted to the device. This would not require the solvent-exposed use of non-membrane sealing components. It would also eliminate the use of potting materials. However, attempts to seal sulfonated halopolymer membranes by permanently forming them into their desirable positions have been unsuccessful. Sulfonated halopolymers are generally considered to be incapable of being effectively deformed to a permanent shape because of their elastic properties at room temperature and because thermal decomposition begins before the sulfonated halopolymers become fluid enough for deformation at an elevated temperature. See R. B. Moore et al., "Barriers to Flow in Semicrystalline Ionomers," 75 J. Membrane Sci. 8 (1992).